Ultrafast sub-threshold photo-induced response in crystalline and amorphous GeSbTe thin films

Shu, Michael J.; Chatzakis, Ioannis; Kuo, Yu‐Hsuan; Zalden, Peter; Lindenberg, Aaron M.

doi:10.1063/1.4807731

Cited by 28 publications

(35 citation statements)

References 34 publications

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“…Thus we conclude that ρ 2 is dominated by the response of the free carriers. Also, the correspondence in the timescales of free carriers measured in the THz regime to that seen here further supports a Drude interpretation 26 . This makes the response of GSTa similar to that of amorphous GeSb 31 .…”

Section: Discussionsupporting

confidence: 70%

“…In the amorphous state the recovery is faster, ranging from 0.8 ps at low fluences to 1.2 ps at 4 mJ cm −2 . As this term contains the equilibration between excited carriers and the lattice, the faster thermalization of GSTa is consistent with observations of carrier lifetime decay using THz spectroscopy 26 . The remaining fit parameters describe how the electronic and crystallographic response (the a and b terms) modify the dielectric function.…”

Section: Fig 4 (Color Online) Schematic Representation Of the Modelsupporting

confidence: 71%

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Ultrafast optical response of the amorphous and crystalline states of the phase change materialGe2Sb2Te5

et al. 2016

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We examine the ultrafast optical response of the crystalline and amorphous phases of the phase change material Ge2Sb2Te5 below the phase transformation threshold. Simultaneous measurement of the transmissivity and reflectivity of thin film samples yields the time-dependent evolution of the dielectric function for both phases. We then identify how lattice motion and electronic excitation manifest in the dielectric response. The dielectric response of both phases is large but markedly different. At 800 nm, the changes in amorphous GST are well described by the Drude response of the generated photo-carriers, whereas the crystalline phase is better described by the depopulation of resonant bonds. We find that the generated coherent phonons have a greater influence in the amorphous phase than the crystalline phase. Furthermore, coherent phonons do not influence resonant bonding. For fluences up to 50% of the transformation threshold, the structure does not exhibit bond softening in either phase, enabling large changes of the optical properties without structural modification.

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Section: Discussionsupporting

confidence: 70%

Section: Fig 4 (Color Online) Schematic Representation Of the Modelsupporting

confidence: 71%

Ultrafast optical response of the amorphous and crystalline states of the phase change materialGe2Sb2Te5

et al. 2016

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“…Here we demonstrate how ultrashort laser pulses can be employed to study the crystallization of PCMs and at the same time overcome earlier limitations on the heating rates. Optical excitation with femtosecond laser pulses heats the lattice through relaxation of photoexcited carriers on the picosecond timescale 19 and corresponds to heating rates around 3*10 14 K/s.…”

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confidence: 99%

How Supercooled Liquid Phase-Change Materials Crystallize: Snapshots after Femtosecond Optical Excitation

et al. 2015

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Glass forming materials are employed in information storage technologies making use of the transition between a disordered (amorphous) and an ordered (crystalline) state. With increasing temperature the crystal growth velocity of these phase-change materials becomes so fast that prior studies have not been able to resolve these crystallization dynamics. However, crystallization is the time limiting factor in the write speed of phase-change memory devices. Here, for the first time, we quantify crystal growth velocities up to the melting point by using the relaxation of photo-excited carriers as an ultrafast heating mechanism.During repetitive femtosecond optical excitation, each pulse enables dynamical evolution for tens of picoseconds before the intermediate atomic structure is frozen-in as the sample rapidly cools. We apply this technique to Ag 4 In 3 Sb 67 Te 26 (AIST) and compare the dynamics of as-deposited and application-relevant melt-quenched glass. Both glasses retain their different kinetics even in the supercooled liquid state, thereby revealing differences in their kinetic fragilities. This approach enables the characterization of application-relevant properties of phase-change materials up to the melting temperature, which has not been possible before.Mankind has utilized glasses during the last five thousand years. They can be prepared by cooling a liquid fast and far enough below the glass transition temperature -to a temperature where its viscosity is sufficiently high that the atomic arrangement is kinetically frozen-in 1 . Until recently, research and technology have focused on good glass formers, i.e. materials which can be vitrified by cooling their liquid state at moderate rates. But in recent decades poor glass formers such as metallic glasses and certain chalcogenide glasses have gained interest due to their remarkable property portfolio 2,3 . These materials need to be cooled at rates in excess of around 3*10 9 K/s to bypass crystallization and to quench the atoms in an amorphous arrangement 4 . This so-called glass transition at temperature is commonly observed at a

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“…This assumption leads to a calculated lattice temperature using the thermal expansion coefficient of 1.74 × 10 −5 K −1 (ref. 26) of ≈930 K, a temperature which is higher than the melting point, which is inconsistent with observation and thus indicates the presence of long-lived non-thermal effects27.…”

Section: Discussionmentioning

confidence: 73%

Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials

Mitrofanov

Fons

Makino

et al. 2016

Sci Rep

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Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived (>100 ps) transient metastable state of Ge2Sb2Te5 with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies.

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Ultrafast sub-threshold photo-induced response in crystalline and amorphous GeSbTe thin films

Cited by 28 publications

References 34 publications

Ultrafast optical response of the amorphous and crystalline states of the phase change materialGe2Sb2Te5

Ultrafast optical response of the amorphous and crystalline states of the phase change materialGe2Sb2Te5

How Supercooled Liquid Phase-Change Materials Crystallize: Snapshots after Femtosecond Optical Excitation

Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials

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